The present invention relates to a mask blank necessary to form, for example, a photomask and a reticule, and particularly, to modification of the shape of the mask blank.
In this kind of a conventional mask blank, a chrome (Cr) layer is formed as a shading film on a surface of a square substrate of quartz. When a pattern is formed as, for example, a photomask on this mask blank, photoresist is first applied to an entire surface of the mask blank and patterned. With the patterned resist used as a mask, the shading film is processed in, for example, isotropic etching and the resist pattern is transferred to the shading film. The pattern is exposed to a wafer by use of the photomask thus formed.
When resist is applied to the mask blank, a spin coater is used similarly to a case where resist is applied to a wafer. However, when resist is applied to a square mask blank by use of a spin coater, it is difficult to apply the resist with a uniform thickness on an entire surface of the mask blank. That is, on a rotated substrate, the resist does not spread uniformly and omnidirectionally by the centrifugal force and concentrates at angular portions of the substrate. For this reason, the film thickness of the resist at the angular portions is larger than that at a central portion on the substrate.
FIG. 7 shows a state of resist 72 applied to a surface of a conventional mask blank 71 and FIG. 8 shows distribution of the film thickness of the resist 72 as seen along a line 8--8 of FIG. 7. As shown in FIG. 8, the film thickness of the resist can be substantially uniform at the central portion of the mask blank 71, but the film thickness of the resist in the vicinity of the angular portions is larger than that at the central portion. If the film thickness of the resist is non-uniform as mentioned above, it is difficult to exactly transfer a pattern on the resist and impossible to exactly form a resist pattern. Therefore, if the shading film is etched by use of this resist as a mask, accuracy in the size of the pattern formed on this shading film is also worsened. Further, the photomask thus formed is mounted on a stepper and the pattern is projected at a reduced size on a wafer. Recently, a semiconductor device has been very miniaturized. Therefore, if the photomask pattern has an error in accuracy of the size, the pattern formed on a semiconductor wafer also has an error as shown in FIG. 9, and a CD (critical dimension) of the formed pattern is worsened. On the other hand, it is impossible to form the pattern in a region where the film thickness of the resist is non-uniform, and the region where the pattern can be formed on the mask blank is limited to an area represented by W in FIG. 8. For this reason, there is a problem that the design margin in pattern formation is decreased.